The catalog pg_listener supports
the LISTEN and NOTIFY commands. A listener creates an entry in
pg_listener for each notification
name it is listening for. A notifier scans pg_listener and updates each matching entry to
show that a notification has occurred. The notifier also sends a
signal (using the PID recorded in the table) to awaken the
listener from sleep.

Table 43-20. pg_listener Columns

Name

Type

References

Description

relname

name

Notify condition name. (The name need not match any
actual relation in the database; the name relname is historical.)

listenerpid

int4

PID of the server process that created this
entry.

notification

int4

Zero if no event is pending for this listener. If an
event is pending, the PID of the server process that sent
the notification.